The invention relates to a photomultiplier tube, and particularly to a structure for isolating the active electronic elements of the cage assembly of such a tube from mechanical stress induced by environmental factors.
The growing demand for petroleum products has placed increasing emphasis upon the need for improved oil exploration techniques. As the supply of easily obtainable oil dwindles, exploration has had to move to more remote geographical areas and to deeper fields. One technique for accurately determining the location, size and yield of an oil field is by oil-well logging. Logging is a term given to the method of determining the mineral composition and structure of the geological media along very deep holes. Sensitive probes, or sondes, are used to determine the lithology, i.e., the character of the rock formation, including the density, of the media along the bore hole. The bore holes are typically thousands of feet deep and may extend to about twenty thousand feet. Temperature increases with bore hole depth and the temperature in a twenty thousand feet deep hole may range between 100.degree. to 250.degree. C. In logging such a hostile environment, the sondes, which include a radioactive gamma ray source such as cesium 137 and a detector comprising a sodium iodide crystal and a photomultiplier tube, are subjected to shock and vibration in addition to high operating temperatures.
Gamma rays from the cesium 137 source enter the medium surrounding the bore hole, and interactions occur among the gamma rays and the orbital electrons in the atoms of the material comprising the medium. The interactions impart energy to the orbital electrons and redirect or scatter photons of lower energy than the incident gamma ray in a direction different from that of the incident gamma ray. This effect is called the Compton Effect. Some of the scattered photons are detected by the sodium iodide crystal which converts them to luminous scintillations. The luminous scintillations are then detected and converted into electrical pulses by the photomultiplier tube. The electrical pulses represent Compton photon energy data which may then be converted into a geological formation-density log. A more complete description of oil-well logging is contained in an article by G. N. Butterwick, entitled, "Oil Exploration With Photomultiplier Tubes", published in the RCA Engineer, pp. 62-65 (Vol. 24, No. 5, February/March 1979).
Photomultiplier tubes used for oil-well logging are preferably small and rugged. The RCA C33016G photomultiplier tube, shown in FIG. 1, has a 25.4 mm diameter and a length of about 60 mm. It is well known in the art that the deleterious effects of shock and vibrations can be minimized by using stiff, short support leads connected directly to the active tube elements such as the anode and the dynodes to resist and quickly damp vibrations. Unfortunately, in photomultiplier tubes such as the C33016G, which has small electron multipliers with typical interelectrode spacings of about 0.1 mm between adjacent active multiplier elements, the stiff support leads expand as the sondes are lowered into the bore hole and the temperature increases and sometimes cause electrical shorts between the closely spaced active tube elements.